This invention relates to a magnetic recording medium, and in particular to a longitudinal magnetic recording medium having a high recording density.
A magnetic recording medium in which a film of Cr, W or Mo or an alloy film consisting essentially of one of these metals is formed, by vapor deposition or sputtering, as a sublayer on a non-magnetic substrate, and in which a Co film or a Co-alloy film is formed on top of the sublayer, is widely used as a hard disc medium capable of high-density recording. In this hard disk recording medium, the memory is reproduced by a magnetic head flying over the recording medium which is rotated at a high speed. In this connection, it is known that if the flying height of the magnetic head can be reduced, the recording density is consequently increased.
Conventional hard disc substrates which are widely used are: a metallic substrate such as an Al-substrate having a nonmagnetic metallic plating thereon, or an "ALMITE"-substrate, or a glass substrate.
By performing a so-called texturing treatment so as to provide a multitude of fine concentric grooves on the surface of the metallic substrate, the coefficient of friction of the recording medium is made smaller as a result of decreasing the effective area of contact between the substrate and the magnetic head. In addition, it is common practice to be able to manufacture with a metallic substrate a disk having circular magnetic anisotropy in the magnetic layer characterized by good squareness of the hysteresis loop in the circular direction.
However, when a metallic substrate is used, the disk surface cannot be made completely smooth due to protrusions, minute undulations, or other irregularities attributable to the presence of intermetallic compounds or defects. Therefore, when the flying height of the magnetic head is reduced, a so-called phenomenon of head crushing occurs in which the magnetic head and the protrusions on the disk collide with each other, resulting in breakage of the magnetic recording medium. Even in the case where the above-mentioned texture-treated substrate is used, this kind of undulations and protrusions cannot be completely removed and, consequently, head crushing is also likely to occur. Therefore, with a magnetic recording medium using a metallic substrate, the flying height of the magnetic head cannot be made small enough, consequently limiting the extent to which the recording density could be increased.
On the other hand, when a glass substrate is used, greater smoothness can be obtained than when a metallic substrate is used. However, the magnetic head and the disk tend to adhere to each other, resulting in a high coefficient of friction and, in an extreme case, the magnetic head and the disk become stuck together so that the disk can no longer be rotated. In order to lower the coefficient of friction, it is also known to texture-treat the glass substrate. In this case, however, because magnetic anisotropy in the circular direction of the disk cannot be obtained, the squareness of the hysteresis loop is poor, which makes the disk unsuitable for high-density recording. Further, when a glass substrate is used, the coercive force of the magnetic recording film which can be obtained is small and, because glass is an electrically insulating body, it cannot be charged with a substrate bias which is one of the means of increasing the coercive force, thus making such a disk unsuitable for high-density recording.
As has been explained above, prior to applicants' invention there was no magnetic recording medium which met all the conditions which would make it suitable for high-density recording.